An implementation of draft-mogul-pps-api-05.txt for Linux 2.2.12 is
available as an extension package. The API has been implemented for the serial
driver, using the Carrier Detect (CD) pin. The optional time_pps_kcbind() is
implemented for hardpps(). At the time of writing the most current version is
contained in PPSkit-0.8.0.tar.gz (The packages adds general PPS capabilities,
as well as a kernel upgrade to nanoseconds, the newer NTP algorithms, and the
PPS API).

The PPS-API based on draft-mogul-pps-api-05.txt is a standard part of
FreeBSD. It is included in FreeBSD-CURRENT as of 10 Oct 1999,
FreeBSD-3.3-STABLE as of 16 Oct 1999 and releases built after these dates.
(Until the specification changes.)

The implementation includes driver support for parallel ports and async
serial ports.